Method for removing a sacrificial material with a compressed fluid

ABSTRACT

A method comprises depositing an organic material on a substrate; depositing additional material different from the organic material after depositing the organic material; and removing the organic material with a compressed fluid. Also disclosed is a method comprising: providing an organic layer on a substrate; after providing the organic layer, providing one or more layers of a material different than the organic material of the organic layer; removing the organic layer with a compressed fluid; and providing an anti-stiction agent with a compressed fluid to material remaining after removal of the organic layer.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.10/167,272 to Jason Reid, et al filed Jun. 10, 2002, which claimspriority from a U.S. provisional patent application Ser. No. 60/298,529filed on Jun. 15, 2001, the subject matter of each being incorporatedherein by reference.

BACKGROUND

A wide variety of micro-electromechanical devices (MEMS) are known,including accelerometers, DC relay and RF switches, optical crossconnects and optical switches, microlenses, reflectors and beamsplitters, filters, oscillators and antenna system components, variablecapacitors and inductors, switched banks of filters, resonantcomb-drives and resonant beams, and micromirror arrays for direct viewand projection displays. There are a wide variety of methods for formingMEMS devices, including a) forming micromechanical structuresmonolithically on the same substrate as actuation or detectioncircuitry, b) forming the micromechanical structures on a separatesubstrate and transferring the formed structures to a circuit substrate,c) forming circuitry on one substrate and forming micromechanicalelements on another substrate and bonding the substrates side by side orin a flip-chip type arrangement. Regardless of the actual method used,at some point in the manufacturing process for making MEMS devices, asacrificial layer is generally removed in order to release themicromechanical structure. The released structure is then able to beactively actuated or moved, such as pivoting or rotation of amicromirror for a projection display or optical switch, or movementduring sensing, such as an accelerometer in an automobile airbag system.

SUMMARY OF THE INVENTION

The present invention is directed to a method for releasing amicromechanical structure, comprising providing a substrate; providing asacrificial layer directly or indirectly on the substrate; providing oneor more micromechanical structural layers on the sacrificial layer; andreleasing the one or more micromechanical structural layers by removingthe sacrificial layer with a supercritical fluid. The sacrificial layerpreferably comprises an organic material.

The invention is more particularly directed to a method comprisingdepositing an organic material on a substrate; depositing additionalmaterial different from said organic material after depositing theorganic material; and removing the organic material with a compressedfluid. The invention is also directed to a method comprising: providingan organic layer on a substrate; after providing the organic layer,providing one or more layers of a material different than the organicmaterial of the organic layer; removing the organic layer with acompressed fluid; and providing an anti-stiction agent with a compressedfluid to material remaining after removal of the organic layer. Forperforming such methods, an apparatus can be provided having a chamber,a holder for holding the device to be processed, a source ofsupercritical CO2 connected directly or indirectly to the chamber, asource of solvent connected directly or indirectly to the chamber, and asource of stiction agent connected directly or indirectly to thechamber.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A to 1E illustrate one method for forming micromirrors;

FIG. 2 is a top view of a micromirror showing line 1-1 for taking thecross section for FIGS. 1A to 1E;

FIGS. 3A to 3E illustrate the same method as in FIGS. 1A to 1D but takenalong a different cross section;

FIG. 4 is a top view of a mirror showing line 3-3 for taking the crosssection for FIGS. 3A to 3E;

FIGS. 5 to 7 illustrate a method for making a different type ofmicromirror than that illustrated in FIG. 1-4; and

FIG. 8 is an illustration of the I/O pads and backplane for the mirrorarray of the present invention.

DETAILED DESCRIPTION

Throughout the present application structures or layers are disclosed asbeing “on” (or deposited on), or over, above, adjacent, etc. otherstructures or layers. It should be recognized that this is meant to meandirectly or indirectly on, over, above, adjacent, etc., as it will berecognized in the art that a variety of intermediate layers orstructures could be interposed, including but not limited to sealantlayers, adhesion promotion layers, electrically conductive layers,layers for reducing stiction, etc. In the same way, structures such assubstrate or layer can be as a laminate due to additional structures orlayers. Also, when the phrase “at least one” or “one or more” (orsimilar) is used, it is for emphasizing the potential plural nature ofthat particular structure or layer (particularly for ease of claimdrafting), however this phraseology should in no way imply the lack ofpotential plurality of other structures or layers that are not set forthin this way. In the same way, when the phrase “directly or indirectly”is used, it should in no way restrict, elsewhere where this phrase isnot used, the meaning elsewhere to either directly or indirectly. Also,“MEMS”, “micromechanical” and “micro electromechanical” are usedinterchangeably herein and, in addition to the microscopic (or smaller)mechanical aspect, the structure may or may not have an electricalcomponent. Lastly, unless the word “means” in a “means for” phrase isspecifically set forth in the claims, it is not intended that anyelements in the claims be interpreted in accordance with the specificrules relating to “means for” phraseology.

MEMS Device Fabrication:

Processes for microfabricating a MEMS device such as a movablemicromirror and mirror array are disclosed in U.S. Pat. Nos. 5,835,256and 6,046,840 both to Huibers, the subject matter of each beingincorporated herein by reference. A similar process for forming MEMSmovable elements (e.g. mirrors) on a wafer substrate (e.g. a lighttransmissive substrate or a substrate comprising CMOS or othercircuitry) is illustrated in FIGS. 1 to 4.

FIGS. 1A to 1D show a manufacturing process for one example of amicromechanical (mirror) structure. As can be seen in FIG. 1A, asubstrate such as glass (e.g. Corning 1737F or Eagle2000), quartz,Pyrex™, sapphire, (or silicon alone or with circuitry thereon) etc. isprovided. The cross section of FIGS. 1A-D is taken along line 1-1 ofFIG. 2. Because this cross section is taken along the hinge of themovable element, an optional block layer 12 can be provided to blocklight (incident through the light transmissive substrate during use)from reflecting off of the hinge and potentially causing diffraction andlowering the contrast ratio (if the substrate is transparent).

As can be seen in FIG. 1B, an organic sacrificial layer 14 (made of amaterial comprising a carbon compound) is deposited. The thickness ofthe sacrificial layer can be wide ranging depending upon the movableelement/mirror size and desired tilt angle, though a thickness of from500 Å to 50,000 Å, preferably around 5000 Å is preferred. As will bediscussed in further detail below, a lithography step is performed witha resist (on top of the sacrificial layer), or, if the sacrificial layercomprises a light sensitive material, the sacrificial layer can bedirectly patterned without the need for a separate resist. Either way,holes 16 a,b are formed in the sacrificial organic material, which holescan be any suitable size, though preferably having a diameter of from0.1 to 1.5 um, more preferably around 0.7+/−0.25 um. The etching isperformed down to the glass/quartz substrate or down to the block layerif present. Preferably if the glass/quartz layer is etched, it is in anamount less than 2000 Å.

At this point, as can be seen in FIG. 1C, a first layer 18 is depositedby chemical vapor deposition. Preferably the material is silicon nitrideor silicon oxide deposited by any suitable method such as sputtering,LPCVD or PECVD, however other materials such as polysilicon, amorphoussilicon, silicon carbide or a different organic compound could bedeposited at this point. The thickness of this first layer can varydepending upon the movable element size and desired amount of stiffnessof the element, however in one embodiment the layer has a thickness offrom 100 to 3200 Å, more preferably around 1100 Å. Though the firstlayer can be patterned at this point, it is preferred that the firstlayer be patterned after all the structural layers are deposited (so asto form deflectable elements with gaps between adjacent deflectableelements of from 0.1 to 25 um, preferably around 1 to 2 um.

A second layer 20 (the “hinge” layer) is deposited as can be seen inFIG. 1D. By “hinge layer” it is meant the layer that defines thatportion of the device that flexes to allow movement of the device. Thehinge layer can be disposed only for defining the hinge, or for definingthe hinge and other areas such as the mirror. In any case, it ispreferred that the first layer is removed in hinge areas prior todepositing the hinge material (second layer). The material for thesecond (hinge) layer can be the same (e.g. silicon nitride) as the firstlayer or different (silicon oxide, silicon carbide, polysilicon, etc.)and can be deposited by any suitable method such as sputtering orchemical vapor deposition as for the first layer. The thickness of thesecond/hinge layer can be greater or less than the first, depending uponthe stiffness of the movable element, the flexibility of the hingedesired, the material used, etc. In one embodiment the second layer hasa thickness of from 50 Å to 2100 Å, and preferably around 500 Å. Inanother embodiment, the first layer is deposited by PECVD and the secondlayer by LPCVD.

As also seen in FIG. 1D, a reflective and conductive layer 22 isdeposited. The reflective/conductive material can be gold, aluminum orother metal, or an alloy of more than one metal though it is preferablyaluminum deposited by PVD. The thickness of the metal layer can be from50 to 2000 Å, preferably around 500 Å. It is also possible to depositseparate reflective and conductive layers. An optional metal passivationlayer (not shown) can be added, e.g. a 10 to 1100 Å TiN or TiON layerdeposited by PECVD. Then, photoresist patterning on the metal layer isfollowed by etching through the metal layer with a suitable metaletchant. In the case of an aluminum layer, a chlorine (or bromine)chemistry can be used (e.g. a plasma/RIE etch with Cl₂ and/or BCl₃ (orCl2, CCl4, Br2, CBr₄, etc.) with an optional preferably inert diluentsuch as Ar and/or He).

In the embodiment illustrated in FIGS. 1A to 1D, both the first andsecond layers are deposited in the area defining the movable (mirror)element, whereas the second layer, in the absence of the first layer, isdeposited in the area of the hinge. It is also possible to use more thantwo layers to produce a laminate movable element, which can be desirableparticularly when the size of the movable element is increased such asfor switching light beams in an optical switch. A plurality of layerscould be provided in place of single layer 18 in FIG. 1C, and aplurality of layers could be provided in place of layer 20 and in placeof layer 22. Or, layers 20 and 22 could be a single layer, e.g. a puremetal layer or a metal alloy layer or a layer that is a mixture of e.g.a dielectric or semiconductor and a metal. Some materials for such layeror layers that could comprise alloys of metals and dielectrics orcompounds of metals (particularly the transition metals) and nitrogen,oxygen or carbon are disclosed in U.S. provisional patent application60/228,007, the subject matter of which is incorporated herein byreference.

Whatever the specific combination, it is desirable that thefirst/reinforcing layer(s) is provided and patterned (at least in thehinge area) prior to depositing and patterning the hinge material andmetal. In one embodiment, the reinforcing layer is removed in the areaof the hinge, followed by depositing the hinge layer and patterning bothreinforcing and hinge layer together. This joint patterning of thereinforcing layer and hinge layer can be done with the same etchant(e.g. if the two layers are of the same material) or consecutively withdifferent etchants. The reinforcing and hinge layers can be etched witha chlorine chemistry or a fluorine chemistry where the etchant is aperfluorocarbon or hydrofluorocarbon (or SF6) that is energized so as toselectively etch the reinforcing and/or hinge layers both chemically andphysically (e.g. a plasma/RIE etch with CF₄, CHF₃, C₃F₈, CH₂F₂, C₂F₆,SF₆, etc. or more likely combinations of the above or with additionalgases, such as CF₄/H₂, SF₆/Cl₂, or gases using more than one etchingspecies such as CF₂Cl₂, all possibly with one or more optional inertdiluents). Of course, if different materials are used for thereinforcing layer and the hinge layer, then a different etchant can beemployed for etching each layer. Alternatively, the reflective layer canbe deposited before the first (reinforcing) and/or second (hinge) layer.Whether deposited prior to the hinge material or prior to both the hingematerial and the reinforcing material, it is preferable that the metalbe patterned (e.g. removed in the hinge area) prior to depositing andpatterning the hinge material.

FIGS. 3A to 3D illustrate the same process taken along a different crosssection (cross section 3-3 in FIG. 4) and show the optional block layer12 deposited on the light transmissive substrate 10, followed by thesacrificial layer 14, layers 18, 20 and the metal layer 22. The crosssections in FIGS. 1A to 1D and 3A to 3D are taken along substantiallysquare mirrors in FIGS. 2 and 4 respectively. However, the mirrors neednot be square but can have other shapes that may decrease diffractionand increase the contrast ratio. Also, the hinges need not be torsionhinges but could instead be flexure hinges. Such hinges and mirrors aredisclosed in U.S. provisional patent application 60/229,246 to Ilkov etal., the subject matter of which is incorporated herein by reference,and are disclosed further below.

As can be seen in FIG. 5, a mirror having the shape as illustrated, isformed in accordance with the following. As can be seen in FIGS. 6 a to6 c (taken along cross section 6 in FIG. 5), a substrate 1 (transparentsubstrate such as quartz, sapphire or glass—e.g. Corning 1737 orEagle2000; or a silicon substrate with circuitry and electrodes) isprovided. Not shown on the substrate are optional light blocking,transparent/conductive (e.g. tin oxide, indium oxide), or other layersthat could be added prior to deposition of the sacrificial layer.Sacrificial layer 2 comprises a carbon compound, preferably an organicchemical compound, that is provided preferably by spin-on coating orspray coating. As mentioned above, a separate photoresist can beprovided on the organic layer in order to pattern the organic layer—inthis case to provide holes for mirror posts. Or, if the organic layercomprises a substance that provides a photochemical route for modifyingthe dissolution rate of the organic material in a developer, then theorganic layer can be patterned directly without an additionalphotoresist layer.

Either way, holes 6 a and 6 b are formed in the sacrificial organicmaterial. The removal of organic material in the area of the holes isperformed down to the glass/quartz substrate or down to any intermediatelayers if present. At this point, as can be seen in FIG. 6B, a firstlayer 7 (e.g. amorphous silicon, polysilicon or a silicon compound suchas silicon nitride or silicon dioxide) is deposited by deposited by anysuitable method such as sputtering, LPCVD or PECVD, however othermaterials such as silicon carbide or a different organic compound couldbe deposited at this point. The thickness of this first layer can varydepending upon the movable element size and desired amount of stiffnessof the element.

A second layer 8 (the “hinge” layer) is deposited as can be seen in FIG.6C. By “hinge layer” it is meant the layer that defines that portion ofthe device that flexes to allow movement of the device. The hinge layercan be disposed only for defining the hinge, or for defining the hingeand other areas such as the mirror. In any case, it is preferred thatthe first layer is removed in hinge areas prior to depositing the hingematerial (second layer). The material for the second (hinge) layer canbe the same (e.g. silicon nitride) as the first layer or different(silicon oxide, silicon carbide, polysilicon, etc.) and can be depositedby any suitable method such as sputtering or chemical vapor depositionas for the first layer. The thickness of the second/hinge layer can begreater or less than the first, depending upon the stiffness of themovable element, the flexibility of the hinge desired, the materialused, etc.

As also seen in FIG. 6C, a reflective and conductive layer 9 isdeposited. The reflective/conductive material can be gold, aluminum orother metal, or an alloy of more than one metal though it is preferablyaluminum deposited by PVD. It is also possible to deposit separatereflective and conductive layers. An optional metal passivation layer(not shown) can be added, e.g. a 10 to 1100 Å TiN or TiON layerdeposited by PECVD. Then, photoresist patterning on the metal layer isfollowed by etching through the metal layer with a suitable metaletchant. In the case of an aluminum layer, a chlorine (or bromine)chemistry can be used (e.g. a plasma/RIE etch with Cl₂ and/or BCl₃ (orCl2, CCl4, Br2, CBr₄, etc.) with an optional preferably inert diluentsuch as Ar and/or He).

The reinforcing and hinge layers 7, 8 can be etched with a chlorinechemistry or a fluorine chemistry where the etchant is a perfluorocarbonor hydrofluorocarbon (or SF6) that is energized so as to selectivelyetch the reinforcing and/or hinge layers both chemically and physically(e.g. a plasma/RIE etch with CF₄, CHF₃, C₃F₈, CH₂F₂, C₂F₆, SF₆, etc. ormore likely combinations of the above or with additional gases, such asCF₄/H₂, SF₆/Cl₂, or gases using more than one etching species such asCF₂Cl₂, all possibly with one or more optional inert diluents). Ofcourse, if different materials are used for the reinforcing layer andthe hinge layer, then a different etchant can be employed for etchingeach layer. Alternatively, the reflective layer can be deposited beforethe first (reinforcing) and/or second (hinge) layer. Whether depositedprior to the hinge material or prior to both the hinge material and thereinforcing material, it is preferable that the metal be patterned (e.g.removed in the hinge area) prior to depositing and patterning the hingematerial. FIGS. 7A to 7C illustrate the same process taken along adifferent cross section (cross section 7-7 in FIG. 5).

Organic Sacrificial Layer:

The sacrificial layer comprises an organic material, a carbon compound,that is deposited by, for example, spray-on or spin-coating. In oneembodiment, the organic material is mixed with a solvent and depositedon a substrate. The solvent is preferably any known solvent fordissolving the organic material to be used, such as a supercriticalfluid and/or a volatile organic solvent. The solvent is selected basedon good handling, spinning and film forming properties (for spin onnon-supercritical embodiments). In a preferred embodiment, asupercritical fluid, such as carbon dioxide, along with a cosolvent,dissolves a polymer and deposits the dissolved polymer on a substrate asa sacrificial layer.

The organic material of the sacrificial layer can be any suitableorganic material, selected based on toxicity, type of solvent needed fordissolution, ease of handling, cost, etc. For example, the organiccompound can be, or have a group in its molecule, selected from alkene,cyclic alkene and cyclic alkane, lactone, anhydride, amide, ketal,acetal, acid halide, halide, heterocycle, arene, ozonide, peroxide,epoxide, furan, lactam, aldehyde, detone, alcohol, nitro, hydroxylamine,nitrile, oxime, imine, azine, hydrazone, aniline, azide, ether, phenol,nitroso, azo, diazonium isothiocyanate, thiocyanate, cyanate, etc.Polymers can be used as the organic material—though the greater thecross linking the more likely that an organic solvent should be used asthe supercritical fluid or as a cosolvent in the supercritical fluid.Preferred polymers are alkyds, acrylics, epoxies, fluorocarbons,phenolics, polyimides, polyurethanes, polyvinyls, polyxylylenes andsilicones. Monomers, mixtures of monomers or monomers and polymers canalso be used.

In one preferred embodiment, the organic material for the sacrificiallayer is a photoresist, or photoresist resin. Thought it is notnecessary to use a photoresist resin, there is the benefit that it iseasily commercially available, and fab compatible. Also, if thephotoresist resin is light sensitive or includes a photoactive compound,then the sacrificial layer can be patterned directly, without the needfor a second photoresist for patterning. The photoresist resin can beused on its own (with solvent) or in its commercial embodiment (e.g.polymer/resin, photoacid generator (PAG), additives such as DI,plasticizer, and solvent). Resists, such as cyclized rubber orpoly(chloromethylstyrene) can be used, as can a novolac-based resist, ahydroxystyrene-based resist, a cyclic olefin based resist, anacrylate-based resist or a fluorocarbon-based resist. As will bediscussed further herein, the more crosslinked the resist (or otherorganic material) is, the more likely that a cosolvent will be desirableat the time of removal of the resist (or other organic material). Thepolymer can be made sensitive to light at a particular wavelength by theaddition of a compound or by altering the polymer structure. Forexample, the novolac resist can be mixed with diazonaphthoquinone (DNQ)so that, upon exposure to, e.g. 365 nm light, the DNQ dissolutioninhibitor is converted into a base-soluble acidic photoproduct thatincreases the dissolution rate of the novolac matrix in the exposedregions. The patterning of the sacrificial layer in the presentinvention, such as the formation of holes 16 a, 16 b in FIG. 1B, can beaccomplished by masking the sacrificial layer in all areas except forthe areas corresponding to holes 16 a, 16 b. Then, as will be discussedfurther herein, the holes are formed by use of a standard novolac/DNQdeveloper, or with a supercritical fluid with cosolvent (the cosolventcan be the same as the off-the-shelf developer, though dissolved in thesupercritical fluid). As will be discussed further herein, thesacrificial layer can eventually be removed by using anothersolvent/developer, or with a supercritical fluid and optionalcosolvent—or, preferably, when it is time to remove the remainingsacrificial material, the remainder is also exposed to 365 nm light andremoved in an atmosphere of supercritical fluid (e.g. CO₂) andnovolac/DNQ developer.

Chemically amplified polyhydroxystyrene (PHOST) polymers can also beused. The HOST polymer backbone has protecting groups that becomedeprotected when a photoacid generator (PAG) decomposes when exposed to248 nm wavelength light (e.g. from an ArF excimer laser). Thedeprotection mechanism causes a polarity change in the resist polymer(from lipophilic to hydrophilic) making exposed regions soluble in adeveloper such as tetramethyl-ammonium hydroxide (TMAH). The acidgeneration results from the light exposure, whereas the acid-catalyzedreactions take place during a post-exposure bake (PEB). The use of apolar solvent, such as alcohol or aqueous base results in the generationof positive-tone images, whereas development with a nonpolar organicsolvent such as anisole provides negative-tone images. As mentionedabove, the sacrificial layer can be patterned such as to form holes 16 aand 16 b, with the remainder later exposed to 248 nm light in order tobe removed in an atmosphere of a supercritical fluid and developer (e.g.TMAH). Likewise, a 193 nm photoresist (e.g. an acrylic or cyclic olefinpolymer) could be used, where initial patterning and later removal areaccomplished upon exposure to 193 nm wavelength light (e.g. from a KrFexcimer laser). Other resists that have been used at 248 nm and 193 nm,such as polymethacrylates (e.g. poly(methyl methacrylate)), novolacresists, acrylic acid copolymers or alternating copolymers ofstyrene-maleic anhydride half ester (with aliphatic diazoketones andother dissolution inhibitors). Also usable are alternating copolymers ofnorbornene derivatives with maleic anhydride prepared by radicalpolymerization and polymers consisting of substituted norbornene repeatunits with a transition metal catalyst. Other examples for thesacrificial material are maleic anhydride-cyclic olefin alternatingcopolymers, and poly(norborene-alt-maleic anhydride). Other chemicallyamplified resists that are composed of a polymer resin that is verysoluble in an aqueous base developer, a protecting t-BOC group used toslow down the dissolution of the polymer, photo-acid generators andoptional dyes and additives along with the casting solvent (or suchpolymer resin and casting solvent alone), could also be used.

Negative photoresists, namely those photosensitive films that becomeinsoluble in solvents or water-based developers upon exposure toradiation, can also be used for the sacrificial layer. Preferred areorganic materials that use photoinitiators that can generate freeradicals or strong acids to facilitate polymeric cross-linking or thephotopolymerization of monomeric or oligomeric species. Without anincrease in molecular weight, negative patterns can be achieved by thephotochemical formation of hydrophobic or hydrophilic groups whichprovide preferential solubility between the exposed and unexposed resistfilm. Another way to increase molecular weight is by cationicallypolymerizing monomers such as epoxies and vinyl compounds, or byenabling condensation reactions between phenol formaldehyde resins andamino-based cross-linkers. Changes in polarity can be achieved throughthe acid-catalyzed deprotection of a variety of esters. Two negativephotoresist examples include Shell Chemical EPON resin SU-8 and ShipleyNegative Resist SNR 248.

Other specific examples of commercially available photoresists include ag-Line photoresist (e.g. Shipley Megaposit SPT3000), an i-Linephotoresist (e.g. Shipley Megaposit SPR220 or SPR350) or a DUVphotoresist (e.g. Shipley UVI 10 Series DUV). In a simple form, thephotoresist sacrificial layer is a single organic component materialsuch as PMMA (poly(methyl methacrylate). The photoresist can be anorganic compound and a photocactive compound, such as DNQ-novolacphotoresist (regardless of whether the photoresist sacrificial layerwill be directly patterned. If the photoresist sacrificial layer willnot be directly patterned (and an additional mask or photoresist layeris used for patterning the sacrificial layer), then thediazonaphthoquinone is not needed and a novolac resin can be used alone(e.g. a novolac made from a feed that is a mixture of meta-cresol,para-cresol and other additives as known in the art). Preferably, thenumber average molecular weight of the novolac is between 1000 and 3000,and the weight average molecular weight is preferably 20,000 or less. Itis also possible to use a photoresist that includes a dye, such asShipley SPR-3617, so as to allow for optical monitoring of the removalof the sacrificial layer.

In a particularly preferred embodiment, the sacrificial material is anorganosilicon or fluorinated polymer, such as, though not limited to,157 nm photoresists. Many fluorinated hydrocarbons have the ability todissolve in supercritical carbon dioxide without the need for acosolvent, or with much lower amounts of such cosolvents. Fluoropolymerscan also be made and/or deposited on the wafer substrate insupercritical carbon dioxide, thus allowing for a “greener” method ofmaking, depositing, patterning and removing the sacrificial material. Inaddition, if a separate photoresist is not used for patterning thesacrificial layer, a fluorocarbon photoresist material could be directlypatterned with a fluorine excimer laser (e.g. a VUV 157 nm GAM laser,Lambda-Physik Novaline F1030-1000 Hz 157 nm fluorine laser, or a CymerELX-6500 1000 Hz 157 nm fluorine laser). Fluorinated versions ofcommercial photoresists (e.g. fluorinated novolacs, methacrylates) aswell as fluorinated-based acrylates, and fluorinated-based norbornyl andmaleic anhydride copolymers can be used. For example, fluorinated octylmethacrylate copolymers could be used, as well aspoly(chlorotrifluoroethylene), fluorinated poly(methylmethacrylate),fluorinated styrenes, poly(vinylidene fluoride),polyhexafluoropropylene, poly(tetrafluoroethylene), copolymers fromt-butyl methacrylate and pentafluoropropyl methacrylate, afluoroacrylate polymer, and polyfluoropolyether graft copolymers.Fluoropolymers can be used alone and unmodified (spin coated anddeveloped in liquid or supercritical CO₂) or with a photoacid generatorand acid labile leaving groups. Block copolymers where one of thecomponents is fluorinated (e.g. a block copolymer comprising polystyreneand poly(1,1-dihydroperfluorooctyl acrylate).

In addition to fluoropolymers, siloxane based materials (silicones) aredesirable for use as the sacrificial material of the present invention.Silicones are a class of polymers that have a repeating Si—O backbonewith organic functional groups attached to the Si via Si—C bonds.Examples include poly(dimethyl siloxane)polymers and copolymers, andcopolymers of t-butyl methacrylate and3-methacryloxypropylpentamethyldisiloxane. Silicones(polyorganosiloxanes) are desirable for use in the present invention notonly for their solubility in carbon dioxide, but they can be directlypatterned (exposed to, for example, 400 nm light to photooxidize apattern in the silicone, followed by etching areas in the silicone filmthat are not photooxidized. Siloxanes and fluoropolymers such asdisclosed in U.S. Pat. Nos. 6,174,631 and 6,096,460 both to French etal. (du Pont), and fluoropolymers such as disclosed in WO 017712 and WO0067072 both to Feiring et al. (du Pont), can be used as the sacrificiallayer in the present invention (each of these du Pont references beingincorporated herein by reference). Examples include a) afluorine-containing polymer comprising a repeat unit derived from atleast one ethylenically unsaturated compound containing a fluoroalcoholfunctional group, b) a fluorine-containing copolymer comprising a repeatunit derived from at least one ethylenically unsaturated compound, whereat least one ethylenically unsaturated compound is polycyclic and atleast one ethylenicaly unsaturated compound contains at least onefluorine atom covalently attached to an ethylenically unsaturated carbonatom, c) a fluorine containing copolymer having a repeat unit derivedfrom at least one polycyclic ethylenically unsaturated compound havingat least one atom or group selected from the group consisting offluorine atom, perfluoroalkyl group and perfluoroalkoxy group, d)organosilicates containing aromatic groups (e.g. derived from benzeneand phenanthrene), optionally diluted with an organosilicate diluentmolecule, e) a fluorocarbon blend (such as a polysilicate fluorocarbonmixture), and e) polysiloxane polymers and polysiloxane polymers dopedwith a chromophore. In the present invention, fluorinated polymers,polysiloxanes and organosilicates are preferred for their ability to bedissolved to varying degrees in pure liquid, sub-, near- andsuper-critical carbon dioxide or in the same with only small amounts ofcosolvents.

The organic material for the sacrificial layer can be deposited by spincoating, as is known in the art for depositing photoresist. It is alsopossible to deposit a polymer using chemical vapor deposition (CVD). Inone embodiment of the invention, pulsed plasma enhanced CVD is used todeposit a polymer film, and in another embodiment of the inventionpyrolytic CVD is used. Amorphous cross-linked networks can be depositedin this way, as can specific linear perfluoroalkyl chains (bulkpoly(tetrafluoroethylene). Both organosilicon and fluorocarbon thinfilms can be deposited by chemical vapor deposition. Continuous PECVD(plasma enhanced chemical vapor deposition) can be used, though itresults in greater crosslinking sites than pulsed PECVD or pyrolyticCVD. After CVD deposition, the organic sacrificial material can be bothpatterned and ultimately removed with a supercritical fluid such ascarbon dioxide (with an optional cosolvent), or patterned in atraditional (wet chemical) manner, but removed with a supercriticalfluid, with or without an additional cosolvent. CVD of fluorocarbons isdisclosed, for example, in Smith et al. “Thin Teflon-Like Films forEliminating Adhesion in Released Polysilicon Microstructures”, SandiaNational Laboratories, Albuquerque, N. Mex., the subject matter of whichis incorporated herein by reference. Also, chemical vapor deposition ofpolymer films is disclosed in U.S. Pat. Nos. 5,888,591, 6,156,435, and6,153,269 all to Gleason et al., the subject matter of each beingincorporated herein by reference. Pyrolytic chemical vapor deposition(of silicone films) is disclosed in U.S. Pat. No. 6,045,877 to Gleasonet al., also incorporated herein by reference. In addition to patterningat 157 nm with a fluorine laser, as mentioned hereinabove, afluorocarbon sacrificial layer can be patterned with electron beamlithography, such as disclosed in Stritsman and Ober, Patterning of CVDFluorocarbon Resist Using Electron Beam Lithography and SupercriticalCO2 Development, Cornell University, Cornell Nanofabrication Facility,National Nanofabrication Users Network, p. 32, the subject matter ofwhich being incorporated herein by reference.

In the alternative to CVD, the organic sacrificial layer can bedeposited with a liquified gas, such as liquid CO2, or withsupercritical CO2. Polymers, particularly fluorinated polymers can beformed as a film on a solid substrate using dip-coating with liquidcarbon dioxide as the solvent. Also, liquid carbon dioxide can be as thesolvent for spin coating photoresist (e.g. fluorocarbon photoresist) ona substrate (glass, silicon, etc.). Or, supercritical CO2 could be usedinstead of liquid CO2 in a RESS (Rapid Expansion of SupercriticalSolution) to form a particulate coating as the sacrificial layer.

In order to use liquid CO2 as a solvent for spin coating, ahigh-pressure coating chamber with rotating chuck is useful. To spincoat from liquid CO2, the resist should be soluble in liquid CO2, orsoluble in whatever supercritical fluid is used (alone or with acosolvent). An example of liquid CO2 as a medium for spin coating isdislcosed in Kendall et al. “Liquid Carbon Dioxide Spin Coating Processfor Deep-UV Photoresists”, conference paper from Fluorine in CoatingsIII, Orlando, Fla. 25-27 Jan. 1999, paper 34; and Hoggan et al. “SpinCoating and Photolithography using Liquid and Supercritical CarbonDioxide”, conference paper from ACS, New Orleans, La. 22-26 Aug. 1999,pp 47-8, both being incorporated herein by reference. Without acosolvent, fluorinated polymers are preferred for their solubility inliquid CO2. For example, a fluorinated octyl methacrylate copolymer canbe dissolved in liquid CO2 and spin coated in a high-pressure spincoating apparatus. And, if a photoacid generator is used in conjunctionwith the fluorinated polymer, the photoacid generator can be fluorinatedto improve solubility in the liquid CO2 (e.g.2-perfluorohexyl-6-nitrobenzyl tosylate).

Instead of liquid CO2, supercritical CO2 can be used in a RESS process.In RESS, a product in, for example a supercritical carbon dioxidesolution, which product is provided for the RESS process or ischemically formed upstream, is deposited as micron sized particles byrapid expansion through a nozzle. The solution is preferably heated toaround 80 degrees C. before expansion and/or the pressure is reduced to70 bar, where the product is insoluble. The expansion nozzle may be ashort length of stainless steel capillary or a fine hole cut by laser ina stainless steel plate. The flow may be supersonic or subsonic in thenozzle. In the RESS process, non-volative solutes are dissolved in asupercritical fluid, which results in a solute laden supercriticalphase. A relatively small change in pressure of the supercritical phasecan lead to a large decrease in solvent density, and hence solventpower. By the rapid expansion, or depressurisation, of the supercriticalsolution, a high supersaturation can be obtained. This highsupersaturation leads to high nucleation rates and the precipitation ofvery fine particles when the solution is rapidly expanded through thenozzle. The rapid expansion creates uniform conditions within thenucleating medium so that the precipitated particles have a narrowparticle size distribution. Particle size and distribution can becontrolled by manipulating RESS operational parameters such as thegeometric characteristics of the nozzle, pre-expansion temperature andpressure, and expansion temperature and pressure, and the concentrationof the solute in the supercritical solution. See, for example C. J.Chang et. al. “Precipitation of Microsize Organic Particles fromSupercritical Fluids” AIChE Journal Vol. 35, No 11, p 1876, (1989), andD. W. Matson et. al: “Rapid Expansion of Supercritical Fluid Solutions:Solute Formation of Powders, Thin Films, and Fibers” Ind. Eng. Chem.Res, 26, p2298, (1987). A cosolvent can also be used along withsupercritical carbon dioxide, such as disclosed in J. W. Tom et. al.:“Application of Supercritical Fluids in The Controlled Release of Drugs”Supercritical Fluid Engineering Science, Chapter 19, p238, (1993). Anyof the wide variety of types of poly(tetrafluoroethylene), including forexample Teflon AF (family of amorphous copolymers based onbistrifluoromethyl, difluoro, dioxole, and other fluorine containingmonomers), could be deposited with liquid CO₂ or supercritical CO₂ asoutlined above. Other fluorinated polymers, and other polymers with acosolvent incorporated into the polymer backbone, could be deposited asabove. Methods for coating substrates using carbon dioxide are disclosedin U.S. Pat. Nos. 6,165,559, 6,165,560, and 6,200,637, the subjectmatter of each being incorporated by reference, as well as in WO 027544assigned to North Carolina State University. Prior to deposition of thefluoropolymer on the substrate (whether in an atomosphere of liquid orsupercritical CO₂ or by using a traditional solvent), the fluoropolymercan be synthesized in an atmosphere of supercritical CO₂, such as in WO00/68170, U.S. Pat. No. 5,981,673 or U.S. Pat. No. 5,922,833, eachassigned to Univ. of N. Carolina, Chapel Hill, the subject matter ofeach being incorporated herein by reference.

Spin-on organic (or organic-inorganic hybrid) low-k materials can alsobe used for the sacrificial layer of the present invention. Examplesinclude FLARE™ (an organic spin-on polymer for use as a low-k interlayerdielectric), HOSP™ (a spin-on hybrid siloxane-organic polymer),ACCUFLO™T-13EL (an organic polymer in an organic solvent system) 314,214 Spin-On Glass (SOG) series (a family of siloxane polymers),AccuGlass™T-12B Spin On Glass (belongs to the methylsiloxane family ofpolymers) and 311, 211, 111 Spin On Glass (SOG) series (family ofmethylsiloxanes that combine organic groups on an inorganic polymerbackbone), SiLK™ (a spin-on organic polymer deposited using aconventional spin-coater), Cyclotene™ (derived from B-stagedbisbenzocyclobutene monomers), and PTFE spin-on films from W. L. Gore(Elkton, Md.) and CVD PTFE films from SVG Thermco Group. These low-kmaterials can be deposited by spin-on with traditional solvents (e.g.mesitylene, gamma butyrolactone) or in liquid carbon dioxide alone orwith small amounts of cosolvent. The siloxane-based low-k materials areuseful for being able to be deposited and removed with liquid orsupercritical carbon dioxide without the need for cosolvents (or withminimal amounts of cosolvents). Many low-k materials are also beneficialin that they can be directly patterned (no photoresist forpatterning)—see, for example, Weibel G L, Lewis H G P, Gleason K K, OberC K. “Patternable low-k dielectrics developed using supercritical CO₂”,Polymer Preprints, 2000, 41(2), 1838-1839, incorporated herein byreference. Adhesion promoters can also be used both before and afterdepositing the low-k material.

Deposition Solvents:

Solvents are needed for depositing the sacrificial material on thesubstrate, for patterning the material if needed, and eventuallyremoving the sacrificial material in order to release themicromechanical structures. The release, in accordance with the presentinvention, is in a solvent that is a supercritical fluid. As will bediscussed further herein, the supercritical fluid can be selected from awide variety of fluids that can be provided in a supercritical state. Acosolvent is not needed. Examples include, ethylene, xenon, water,toluene, carbon dioxide, nitrous oxide, methanol, n-pentane, ethane,propane, isopropanol, n-butane and ammonia. If supercritical carbondioxide is used, and the organic material of the sacrificial layer is anorganosilicate or a fluoropolymer, the supercritical CO2 can be usedwithout a cosolvent to remove the sacrificial material to release themicromechanical structures (the supercritical CO2 without cosolvent canalso be used for depositing the sacrificial layer (e.g. in a RESSprocess), or liquid CO2 without cosolvent can be used for sacrificiallayer spin-on as disclosed above). Of course, there are manynon-fluorous polymers with high solubility in supercritical CO2—see, forexample, Sarbu et al. “Non-Fluorous Polymers with Very High Solubilityin Supercritical CO₂ Down to Low Pressures”, Nature, vol 405, no. 6783,2000, pp. 165-168, the subject matter of which is incorporated herein byreference. The removal fluid can be a compressed fluid (a category thatincludes supercritical fluids, near-critical fluids, expanded liquids orhighly compressed gases, depending upon temperature, pressure andcomposition)—though fluids in their supercritical state are preferred.

A cosolvent can be used along with the supercritical fluid to increasesolubility of the sacrificial layer when being removed. This cosolventcan be the same solvent as used for spin-on of the sacrificial materialwhen first deposited (or for patterning/developing the sacrificiallayer) if such is performed in the traditional method without asupercritical fluid. If the sacrificial material is a photoresist andcan be directly patterned, then a cosolvent used for removing thesacrificial material in the end to release the micromechanicalstructures may be different. Cosolvents are discussed in more detail inrelation to organic sacrificial material removal—however, such solventscould be used in the traditional manner, or with a supercritical fluid,for deposition of the sacrificial layer.

If the sacrificial layer material is an off-the shelf photoresist, thenthe corresponding developer might be used for depositing the resist onthe substrate, patterning and/or as the cosolvent with the supercriticalfluid. The solvent that can be used with a novolac or novolac-DNQ can beany of a wide variety of known solvents for novolac resins, such asPGMEA (relatively non-toxic), cyclohexanone, acetone, ethyl lactate, NMP(1-methyl-2-pyrrolidinone), diglyme (diethyleneglycol dimethyl ether) or1,2-propanediol monomethylether acetate. The photoresist can beformulated with a polymer loading of from about 15 to 30 percent byweight with respect to the solvent content of the resist solution.

Circuitry:

In the present invention, the circuitry can be formed together on thesame substrate as the microstructures, such as in U.S. Pat. Nos.5,061,049, 5,527,744, and 5,872,046. If the microstructures are notformed monolithically on the same wafer as the circuitry, then a secondsubstrate can be provided having circuitry thereon (or, circuitry couldbe provided on both the first wafer and the replacement substrate ifdesired). If the microstructures are micromirrors, then it may bepreferable to form circuitry and electrodes on a second wafer substratewith at least one electrode electrostatically controlling one pixel (onemicromirror on the first wafer portion of the die) of the microdisplay.The voltage on each electrode on the surface of the backplane determineswhether its corresponding microdisplay pixel is optically ‘on’ or ‘off,’forming a visible image on the microdisplay. Details of the backplaneand methods for producing a pulse-width-modulated grayscale or colorimage are disclosed in U.S. patent application Ser. No. 09/564,069 toRichards, the subject matter of which is incorporated herein byreference.

The display pixels themselves, in a preferred embodiment, are binary,always either fully ‘on’ or fully ‘off,’ and so the backplane design ispurely digital. Though the micromirrors could be operated in analogmode, no analog capability is necessary. For ease of system design, thebackplane's I/O and control logic preferably run at a voltage compatiblewith standard logic levels, e.g. 5V or 3.3V. To maximize the voltageavailable to drive the pixels, the backplane's array circuitry may runfrom a separate supply, preferably at a higher voltage.

One embodiment of the backplane can be fabricated in a foundry 5V logicprocess. The mirror electrodes can run at 0-5V or as high above 5V asreliability allows. The backplane could also be fabricated in ahigher-voltage process such as a foundry Flash memory process using thatprocess's high-voltage devices. The backplane could also be constructedin a high-voltage process with larger-geometry transistors capable ofoperating at 12V or more. A higher voltage backplane can produce anelectrode voltage swing significantly higher than the 5-7V that thelower voltage backplane provides, and thus actuate the pixels morerobustly.

In digital mode, it is possible to set each electrode to either state(on/off), and have that state persist until the state of the electrodeis written again. A RAM-like structure, with one bit per pixel is onearchitecture that accomplishes this. One example is an SRAM-based pixelcell. Alternate well-known storage elements such as latches or DRAM(pass transistor plus capacitor) are also possible. If a dynamic storageelement (e.g. a DRAM-like cell) is used, it is desirable that it beshielded from incident light that might otherwise cause leakage.

The perception of a grayscale or full-color image will be produced bymodulating pixels rapidly on and off, for example according to themethod in the above-mentioned U.S. patent application Ser. No.09/564,069 to Richards. In order to support this, it is preferable thatthe backplane allows the array to be written in random-access fashion,though finer granularity than a row-at-a-time is generally notnecessary.

It is desirable to minimize power consumption, primarily for thermalreasons. Decreasing electrical power dissipation will increase theoptical/thermal power budget, allowing the microdisplay to tolerate theheat of more powerful lamps. Also, depending upon the way themicrodisplay is assembled (wafer-to-wafer join+offset saw), it may bepreferable for all I/O pads to be on one side of the die. To minimizethe cost of the finished device it is desirable to minimize pin count.For example, multiplexing row address or other infrequently-used controlsignals onto the data bus can eliminate separate pins for thesefunctions with a negligible throughput penalty (a few percent, e.g. oneclock cycle for address information per row of data is acceptable). Adata bus, a clock, and a small number of control signals (5 or less) areall that is necessary.

In use, the die can be illuminated with a 200W or more arc lamp. Thethermal and photo-carrier effects of this may result in special layoutefforts to make the metal layers as ‘opaque’ as possible over the activecircuitry to reflect incident optical energy and minimize photocarrierand thermal effects. An on-chip PN diode could be included for measuringthe temperature of the die.

In one embodiment the resolution is XGA, 1024×768 pixels, though otherresolutions are possible. A pixel pitch of from 5 to 24 um is preferred(e.g. 14 um). The size of the electrode array itself is determined bythe pixel pitch and resolution. A 14 um XGA device's pixel array willtherefore be 14.336×10.752 mm.

As can be seen in FIG. 8, the I/O pads (88) can be placed along theright edge of the die, as the die is viewed with pixel (0,0) (89 in FIG.5) at the top left corner. Putting the pads on the ‘short’ (left/right)edge (87) of the die is preferable due to the slightly reduced die size.The choice of whether the I/O should go on the left vs. right edge ofthe die is of little importance since the display controller ASIC maysupport mirroring the displayed image in the horizontal axis, thevertical axis, or both. If it is desired to orient the display with theI/O on the left edge, the image may simply be rotated 180 degrees by theexternal display controller. The electrode voltage during operation is,in the low state 0V and in the high state preferably from 5 to 7 V (or12V or higher in the higher voltage design). Of course other voltagesare possible, though lower actuation voltages are preferred. In oneembodiment the electrodes are metal squares, though other geometries arepossible. Standard CMOS passivation stackup over the electrodes can beprovided.

Supercritical Fluid Release:

Assembly of the micro-electromechanical device, where mechanicalelements are formed on one substrate and circuitry for interacting withthe mechanical components is provided on another substrate, involvesconnecting the two substrates together (e.g. back to back, side by side,or preferably in a flip chip approach). If the micro-mechanical elementscomprise both micromechanical and electrical components (e.g. areprovided monolithically with the, then no assembly of substrates isneeded and the method can proceed directly to wire-bonding and packaging(though after release).

Supercritical Fluid:

In either case, the micro-mechanical elements are preferably firstreleased by removing the sacrificial layer so as to free the MEMSelements (e.g. micromirrors) to move. In accordance with the invention,the organic sacrificial layer (or layers if multiple sacrificial layersare provided on the substrate) is removed with a supercritical fluid (ornear-supercritical fluid). “Supercritical fluids” is the term used todescribe those fluids that have been compressed beyond their criticalpressure and also heated above their critical temperature. Both gases(e.g. carbon dioxide, nitrous oxide) and liquids (e.g. water) aresuitable. More particularly, fluids that can be made into asupercritical fluid state for the present invention, include inorganicgases and organic gases, such as nitrogen, alkanes and preferably loweralkanes (e.g. methane, ethane, propane, butane), or alkenes, preferablylower alkenes (e.g. propylene). Also usable in the present invention aresupercritical xenon, krypton, methanol, ethanol, isopropanol andisobutanol. Supercritical hydrocarbons or fluorocarbons could also beused, as well as partially fluorinated and perfluorinated halocarbons,and highly polar hydrogen bonding solvents. Other examples ofsupercritical fluids that could be used in the present invention includesupercritical ethanol, acetic acid, xenon and ethane, and mixturesthereof.

More than one supercritical fluid can be used (as a mixture), and one ormore cosolvents (discussed below) can also be used with the mixture ofsupercritical fluids. Various supercritical fluids and their criticaltemperatures and pressures are set forth on pages F-64 to F-66 in CRCHandbook of Chemistry and Physics, 68th Edition, 1987-1988 (these pagesincorporated herein by reference). Near supercritical fluids alsodemonstrate solubility, viscosity, density, and behavior characteristicssimilar to supercritical fluids, and can be used, as can subcriticalfluids (herein defined as a fluid below its critical temperature butabove its critical pressure or vice versa), depending upon the fluid,whether there is an additional solvent, and the nature of the organicmaterial being removed.

Solvents:

Solvents (used in their supercritical state or as a cosolvent with asupercritical fluid) can be selected based on their known ability fordissolving the organic material to be removed (or deposited orpatterned). One approach that is used is to divide the Hildebrand'stotal solubility parameter into secondary intermolecularforces—dispersion, dipole-dipole and hydrogen bonding. When plotted in athree dimensional Cartesian coordinate system, each solvent and polymercan be represented by a “region” (see Barton, Allan, Handbook ofSolubility Parameters and Other Cohesion Parameters, CRC Press, Inc., p.8 and p. 141). Some obvious solvent candidates are those that have knownsolubility of particular photoresist materials, such as amyl acetate,butoxyethanol, gamma butyrolactone, cyclohexanone, dichlorobenzene,ethyl lactate, heptanone, mineral spirits, mesitylene, methyl cellusolveacetate, methyl isobutyl ketone, n-methylpyrolidinone, propylene glycolmonomethyl ether acetate, and xylene.

The phase behavior or ternary systems of carbon dioxide and thesolubilities of a large number of compounds in liquid carbon dioxide andsupercritical carbon dioxide have been much studied since 1954. Carbondioxide is not a very good solvent for high molecular weight and polarcompounds (with some exceptions as noted previously). To increase thesolubility of such compounds in liquid or supercritical carbon dioxide(and subcritical and near supercritical carbon dioxide), small amounts(e.g. less than 50 mol %, preferably from 0 to 25% mol %) of polar ornon-polar cosolvents can be added. These cosolvents can be usedthemselves as the supercritical fluid, however, more environmentallyfriendly substances such as water, carbon dioxide and nitrous oxide arepreferred as the supercritical fluid, with the cosolvent used being aminor mol %. Cosolvents such as methane, ethane, propane, butane, etc.,and methanol, ethanol, propanol, butanol, etc., as well as methylene,ethylene, propylene, butylene, etc., as well as lower hazard organicco-solvents such as methylene carbonate, ethylene carbonate, propylenecarbonate, etc. as well as the chlorides of methylene, ethylene,propylene, etc. can be used. Other possible cosolvents include hexanoicacid, octanoic acid, decanoic acid, pentanoic acid, heptanoic acid,furfural, trioctylamine, isopropylamine, trioctylphosphine oxide,2-ethyl hexanol, n-butanol, n-amyl alcohol, t-amyl alcohol, decylalcohol, and mixtures thereof.

Many other solvents can be used for both depositing the organicsacrificial layer and removing the organic sacrificial layer (as asupercritical fluid or preferably mixed with a supercritical fluid suchas carbon dioxide, water, or nitrous oxide. Examples include ethylacetate, propionitrile, toluene, xylene, tetramethylene sulfone,cellosolve acetate. More particularly, suitable solvents which may beutilized include ketones such as acetone, methyl ethyl ketone, methylisobutyl ketone, mesityl oxide, methyl amyl ketone, cyclohexanone andother aliphatic ketones; esters such as methyl acetate, ethyl acetate,alkyl polycarboxylic acid esters; ethers such as methyl t-butyl ether,dibutyl ether, methyl phenyl ether and other aliphatic or alkyl aromaticethers; glycol ethers such as ethoxy ethanol, butoxy ethanol, ethoxy2-propanol, propoxy ethanol, butoxy propanol and other glycol ethers;glycol ether esters such as butoxy ethoxy acetate, ethyl 3-ethoxypropionate and other glycol ether esters; alcohols such as methanol,ethanol, propanol, iso-propanol, butanol, iso-butanol, amyl alcohol andother aliphatic alcohols; aromatic hydrocarbons such as toluene, xylene,and other aromatics or mixtures of aromatic solvents; aliphatichydrocarbons such as VM&P naphtha and mineral spirits, and otheraliphatics or mixtures of aliphatics; nitro alkanes such as2-nitropropane. A review of the structural relationships important tothe choice of solvent or solvent blend is given by Dileep et al., Ind.Eng. Chem. (Product Research and Development) 24, p. 162 (1985) andFrancis, A. W., J. Phys. Chem. 58, p. 1099 (1954).

If the organic sacrificial layer is an off-the-shelf photoresist, thanthe corresponding commercial developer can be used (mixed with thesupercritical fluid). Well-known solvents used to dissolve acid sensitvephotoresist include ethers, glycol ethers, aromatic hydrocarbons,ketones, esters and the like. One example of an ester that could be usedas the solvent is ethyl lactate, whereas one example of a specificglycol ether being propylene glycol monomethylether acetate (PGMEA). Ifthe organic sacrificial layer is comprised of a novolac or novolac-DNQresin, then an aqueous alkaline solvent such as a metal hydroxide (KOH,NaOH) could be used. Preferably, however, the solvent is an organicnon-metal solvent such as tetramethyl ammonium hydroxide (TMAH).

In one embodiment, the cosolvent used with the supercritical fluid, orused as the supercritical fluid itself, is a fluorinated solvent or asiloxane or siloxane modified solvent. Preferably the fluorinatedsolvent has low viscosity, low cohesive energy density and low sufacetension. Fluorinated solvents that can be used alone or as cosolvents,include hydrofluoroethers, highly fluorinated hydrocarbons, andperfluorohexane. In another embodiment, the cosolvent is a gas used forsilicon etching, such as SF6 or CHF3.

The apparatus for removing the sacrificial layer (and optionallypatterning the sacrificial layer and treating for stiction) can besimilar to the Los Alamos SCORR (Supercritical Carbon diOxide ResistRemover) or GT Equipment's Supercritical CO₂ MEMS Dryer, such asdisclosed in U.S. Pat. No. 6,067,728, incorporated herein by reference.By changing operating parameters, the apparatus can be switched betweenliquid and supercritical carbon dioxide, thus allowing for depositionand removal of organic material in the same machine. Preferred is anapparatus that allows turbulent yet uniform flow through the reactionchamber, and an apparatus with a closed loop system for recirculatingcarbon dioxide and cosolvent but separating out waste.

Assembly:

Releasing immediately prior to the application of epoxy or other bondingis preferable (though an anti-stiction treatment or other passivationtreatment (or treatment for improving epoxy bond strength) betweenrelease and bonding may be desirable). After releasing themicromechanical structures, the remainder of the device can be treatedfor stiction by applying an anti-stiction layer (e.g. a self assembledmonolayer). The layer is preferably formed by placing the device in aliquid or gas silane, preferably a halosilane, and most preferably achlorosilane. Of course, many different silanes and other materials areknown in the art for their ability to provide anti-stiction for MEMSstructures. The anti-stiction material can be appllied with standardprocesses, or even with a compressed fluid (e.g. supercritical nitrousoxide or carbon dioxide).

After releasing the micromechanical structure(s), the first wafer withsuch structures thereon can be packaged (e.g. if circuitry is providedon the first wafer), or the first wafer can be bonded to another waferhaving circuitry thereon, in a “flip-chip” type of assembly. The bondingof the circuitry wafer to the first wafer holding the microstructurescan be by anodic bonding, metal eutectic bonding, fusion bonding, epoxybonding, or other wafer bonding processes known in the art. A preferredbonding method is bonding with an IR or UV epoxy such as disclosed inU.S. Pat. No. 5,963,289 to Stefanov et al, “Asymmetrical Scribe andSeparation Method of Manufacturing Liquid Crystal Devices on SiliconWafers”, which is hereby incorporated by reference. In order to maintainseparation between the bonded wafers, spacers can be mixed into theepoxy. The spacers can be in the form of spheres or rods and can bedispensed and dispersed between the first wafer and sealing wafer inorder to keep the sealing wafer spaced away from the first wafer (so asto avoid damage to the microstructures on the first wafer). Spacers canbe dispensed in the gasket area of the display and therefore mixed intothe gasket seal material prior to seal dispensing. This is achievedthrough normal agitated mixing processes. The final target for the gapbetween the first wafer and sealing wafer can be from 1 to 100 um. Thisof course depends upon the type of MEMS structure being encapsulated andwhether it was surface or bulk micromachined (bulk micromachinedstructures may not need any spacers between the two wafers). The spheresor rods can be made of glass or plastic, preferably an elasticallydeforming material. Alternatively, spacer pillars can be microfabricatedon at least one of the wafer substrates. In one embodiment,pillars/spacers are provided only at the edge of the array. In anotherembodiment, pillars/spacers can be fabricated in the array itself. Ifthe spacers are micro-fabricated spacers, they can be formed on thelower wafer, followed by the dispensing of an epoxy, polymer, or otheradhesive (e.g. a multi-part epoxy, or a heat or UV-cured adhesive)adjacent to the micro-fabricated spacers. The adhesive and spacers neednot be co-located, but could be deposited in different areas on thelower substrate wafer. Alternative to glue, a compression bond materialcould be used that would allow for adhesion of the upper and lowerwafers. Spacers micro-fabricated on the lower wafer (or the upper wafer)and could be made of polyimide, SU-8 photo-resist.

Then, the two wafers are aligned. If precision alignment is desired,alignment of the opposing electrodes or active viewing areas may involveregistration of substrate fiducials on opposite substrates. This taskaccomplished with the aid of video cameras with lens magnification. Themachines range in complexity from manual to fully automated with patternrecognition capability. Whatever the level of sophistication, theyaccomplish the following process: 1. Dispense a very small amount of aUV curable adhesive at locations near the perimeter and off of allfunctional devices in the array; 2. Align the fiducials of the opposingsubstrates within the equipment capability; and 3. Press substrates andUV tack for fixing the wafer to wafer alignment through the remainingbonding process (e.g., curing of the internal epoxy).

The final cell gap can be set by pressing the previously tackedlaminates in a UV or thermal press. In a UV press, a common procedurewould have the substrates loaded into a press where at least one or bothof the press platens are quartz, in order to allow UV radiation from aUV lamp to pass unabated to the gasket seal epoxy. Exposure time andflux rates are process parameters determined by the equipment andadhesive materials. Thermally cured epoxies may require that the top andbottom platens of a thermal press be heated. The force that can begenerated between the press platens is typically many pounds. Withthermally cured epoxies, after the initial press the arrays aretypically transferred to a stacked press fixture where they can continueto be pressed and post-cured. In one embodiment, the epoxy between thefirst wafer and sealing wafer is only partially cured so as to alloweasier removal of the sealing wafer. After the sealing wafer is removed,this epoxy can be optionally cured. An epoxy can be selected thatadheres less well (depending upon the wafer materials) than otherepoxies, so as to allow for easier removal of the sealing wafer aftersingulation. Also, UV epoxy and IR epoxy can be used at the same time,with the UV epoxy being cured prior to IR cure.

Once the wafers have been bonded together to form a wafer assembly, theassembly can be separated into individual dies. Scribes are placed onthe respective substrates in an offset relationship at least along onedirection. The units are then separated, resulting in each unit having aledge on each end of the die. Such a ledge can also allow for electricaltesting of each die, as electrical contacts can be exposed on the ledge(e.g., if circuitry has been formed together with the microstructures onthe first wafer). The parts can then be separated from the array byventing the scribes on both substrates. Automatic breaking can be doneby commercially available guillotine or fulcrum breaking machines. Theparts can also be separated by hand.

Separation may also by done by glass scribing and partial sawing of oneor both substrates. Sawing is preferably done in the presence of ahigh-pressure jet of water. Moisture must not be allowed to contact themicrostructures. Therefore, at gasket dispense, an additional gasketbead must be dispensed around the perimeter of the wafer, or each gasketbead around each die must fully enclose the die area so that water cannot enter and touch the microstructures. Preferably, however, the end ofeach scribe/saw lane must be initially left open, to let air vent duringthe align and press processes. After the array has been pressed and thegasket material fully or partially cured, the vents are then closedusing either the gasket or end-seal material. The glass is then scribedand sawed.

Alternatively, both the first wafer and sealing wafer substrates may bepartially sawed prior to part separation. With the same gasket sealconfiguration, vent and seal processes as described above, saw lanes arealigned to fiducials on the sealing substrate. The glass is sawed to adepth between 25% and 95% of its thickness. The first wafer substrate issawed and the parts separated as described above.

The first wafer, upon which the micromechanical structures are formedand released, can be any suitable substrate for the particular MEMSmicrostructure (and optionally circuitry) formed thereon, such as alight transmissive substrate such as glass, borosilicate, temperedglass, quartz or sapphire, or any other suitable light transmissivematerial. Or, the first wafer could be a metal, ceramic or preferably asemiconductor wafer (e.g. silicon or GaAs).

It should be noted that the invention is applicable to formingmicromirrors such as for a projection display or optical switch, or anyother MEMS. If an optical switch is the microstructure being protected,mirrors with multiple hinges can be provided on the first wafer so as toallow for multi-axis movement of the mirror. Such multi-axis movement,mirrors for achieving such movement, and methods for making such mirrorsare disclosed in U.S. patent application Ser. No. 09/617,149 to Huiberset al., the subject matter of which is incorporated herein by reference.

Of course, the microstructure need not be a movable mirror (for aprojection display, for optical switching, or even for data storage),but could be one or more accelerometers, DC relay or RF switches,microlenses, beam splitters, filters, oscillators and antenna systemcomponents, variable capacitors and inductors, switched banks offilters, resonant comb-drives and resonant beams, etc. Any MEMSstructure, particularly a released or movable structure, could benefitfrom the release method described herein.

It should also be noted that the novel materials used for thesacrificial material can be removed by downstream oxygen plasma release,or with a liquid solvent (flow the liquid solvent across the MEMSstructure/array to remove the organic sacrificial material, followed byflowing an alcohol, freezing the alcohol, and sublimating off thealcohol to release the MEMS structures), or by other methods forremoving organic materials.

The invention has been described in terms of specific embodiments.Nevertheless, persons familiar with the field will appreciate that manyvariations exist in light of the embodiments described herein.

1. A method for making a MEMS device, comprising: providing asacrificial material on a substrate; providing additional materialdifferent from said organic material after depositing the sacrificialmaterial for forming structure for the MEMS device; and removing thesacrificial material with supercritical carbon dioxide so as to releasethe MEMS device.
 2. The method of claim 1, wherein the sacrificialmaterial comprises a photoresist.
 3. The method of claim 2, wherein thephotoresist is a DUV photoresist.
 4. The method of claim 1, wherein thesacrificial material comprises an organic compound.
 5. The method ofclaim 4, wherein the sacrificial material further comprises aphotoactive compound.
 6. The method of claim 1, wherein the sacrificialmaterial comprises a novolac.
 7. The method of claim 1, wherein thesacrificial material comprises a fluorinated hydrocarbon.
 8. The methodof claim 1, wherein the sacrificial material comprises apolyorganosiloxane.
 9. The method of claim 1, wherein the sacrificialmaterial is deposited by spin coating.
 10. The method of claim 1,wherein the sacrificial material is directly patterned.
 11. The methodof claim 1, wherein the sacrificial material comprises a cosolvent forincreasing solubility of the sacrificial material when it is removed.12. The method of claim 1, wherein the sacrificial material is anovolac-DNQ sacrificial material.
 13. The method of claim 1, wherein thesacrificial material comprises a polymer.
 14. The method of claim 1,wherein the sacrificial material is a novolac resin and is depositedwith a cosolvent.
 15. The method of claim 1, wherein the cosolvent isPGMEA, cyclohexanone, acetone, ethyl lactate, NMP(1-methly-2-pyrrolidinone), diglyme (diethyleneglycol dimethyl ether) or1,2-propanediol monomethylether acetate.
 16. The method of claim 1,wherein the sacrificial material is removed with the supercriticalcarbon dioxide and a cosolvent.
 17. The method of claim 16, wherein thecosolvent is methanol.
 18. The method of claim 16, wherein the cosolventis ethanol.
 19. The method of claim 16, wherein the cosolvent ispropanol.
 20. The method of claim 16, wherein the cosolvent is a ketone.21. The method of claim 16, wherein the cosolvent is an acetone.
 22. Themethod of claim 16, wherein the cosolvent is an acetate.
 23. The methodof claim 22, wherein the acetate is methyl acetate.
 24. The method ofclaim 22, wherein the acetate is ethyl acetate.
 25. The method of claim16, wherein the cosolvent is an ether.
 26. The method of claim 25,wherein the ether is methyl t-butyl ether.
 27. The method of claim 20,wherein the ketone is methyl ethyl ketone.
 28. The method of claim 2,wherein the photoresist is patterned at 248 nm.
 29. The method of claim2, wherein the photoresist is patterned at 193 nm.
 30. The method ofclaim 2, wherein the photoresist is patterned at 157 μm.
 31. The methodof claim 1, wherein the sacrificial material is an organic materialpatterned prior to providing the additional material.
 32. The method ofclaim 31, wherein the wherein the patterning comprises directing lightof a particular wavelength at the organic material and removing portionsof the organic material.
 33. The method of claim 1, wherein thesacrificial material is removed with the supercritical carbon dioxideand an organic cosolvent.
 34. The method of claim 33, wherein theorganic cosolvent is an aromatic organic cosolvent.
 35. The method ofclaim 16, wherein the cosolvent is an ester.
 36. The method of claim 16,wherein the cosolvent is a glycol ether.
 37. The method of claim 16,wherein the cosolvent is an aromatic hydrocarbon.
 38. The method ofclaim 1, wherein the additional material comprises a metal.
 39. Themethod of claim 1, wherein after removing the sacrificial material, astiction reducing agent is applied.
 40. The method of claim 39, whereinthe supercritical fluid is the same for removing the sacrificialmaterial as for providing the stiction reducing agent.
 41. The method ofclaim 1, wherein the MEMS device is a micromirror for a display.
 42. Themethod of claim 1, wherein the MEMS device is a micromirror for anoptical switch.
 43. The method of claim 1, wherein the sacrificialmaterial is a fluorocarbon.
 44. The method of claim 1, wherein thesacrificial material is a polyimide.
 45. The method of claim 1, whereinthe sacrificial material is a polyvinyl, polyurethane, acrylic, alkyd orsilicone.
 46. The method of claim 2, wherein the photoresist is anovolac-based resist, a hydroxystyrene-based resist, a cyclic olefinbased resist, an acrylate-based resist or a fluorocarbon-based resist.47. The method of claim 1, wherein the sacrificial material is patternedby masking the sacrificial material in particular areas, followed byexposure and removal of selected areas with developer, followed byproviding the additional material and removing the remaining sacrificialmaterial with the supercritical carbon dioxide.
 48. The method of claim1, wherein the sacrificial material is a resist selected frompolymethacrylates, novolac, acrylic acid copolymers or alternatingcopolymers of styrene-maleic anhydride half ester.
 49. The method ofclaim 1, wherein the additional material comprises silicon or a siliconcompound.
 50. The method of claim 39, wherein the stiction reducingagent is a chlorosilane.
 51. The method of claim 39, wherein thestiction reducing agent is a silane or siloxane.
 52. A method forcoating a MEMS device, comprising: providing a MEMS device; exposing theMEMS device to a supercritical fluid that comprises a decanoic acid.